In a manufacturing process of a semiconductor device including transistors, i.e., MOS transistors, oxide films (hereinafter referred to as “sacrifice oxide film”) may be deposited on surfaces of element regions in order to prevent damage to a semiconductor substrate by well implementation or channel implementation.
The sacrifice oxide film is removed with chemical solution, i.e., hydrofluoric acid (HF) prior to forming a gate insulator. However, a chemical solution such as HF also etches the oxide film for demarcation of the element regions (hereinafter referred as to “buried oxide film”), thus the buried oxide film differs in height after removing the sacrifice oxide film compared to before removing the sacrifice oxide film.
Japanese Laid-open Patent Publication 2001-24055 discloses a planarization technology for the buried oxide film, i.e., a method for leveling the buried oxide film protruded from the surface of the element region to the element region's height. With this approach, firstly, an oxide film (hereinafter referred as to “thermal oxidized film”) is formed on the surface of the element region by thermal oxidation method. Then, the buried oxide film and the thermal oxidized film are coated with photoresist. Thereafter, the thermal oxidized film is exposed by etching the photoresist and the buried oxide film at an even rate. Next, the buried oxide film and the thermal oxidized film are etched by wet etching at the even rate to expose the element regions. Thus the buried oxide film is leveled to the element region in height.
The etching rate of the buried oxide film with the chemical solution such as HF differs according to the type of implemented ion, an amount of the ion's atomic weight or implementation dosage. As such, where plural types of transistors are implemented together, the etching rates of the buried oxide film vary by region of a semiconductor substrate. Thus, where the sacrificed oxide film is removed with the chemical solution such as HF after well/channel implementation, the surface of the buried oxide film may be unleveled.